1. Field of the Invention
The present invention relates to a method for manufacturing a cooling unit and the cooling unit comprising heat pipes for diffusing heat generated from electronic components or the like which has a semiconductor device and others mounted thereon and generates heat.
2. Description of the Related Art
As a means for preventing electronic equipment from overheating, a forced-air cooling system employing an air-cooled fan has been adopted. However, in high-density packaging electronic equipment typified by recent computers, heat generated by the equipment tends to prominently increase because of the high density of heat generating components such as integrated circuits (IC) or large scale integration (LSI) mounted in the equipment, and the cooling system using the air-cooled fan has a limited cooling capability.
Further, with the rapid advance of reducing the size of electronic equipment, a space for mounting the cooling unit becomes smaller within the equipment, which makes heat diffusion in the electronic equipment difficult.
As a countermeasure for solving such problems, there has been proposed a mechanism by which heat generated by electronic components or electronic devices (referred to as electronic components hereinafter) is received by a heat conductor and that heat is then removed from the electronic components. Such a mechanism is partially put into practical use. According to this method, a heat conductive plate or the like is brought into contact with the electronic components which must be cooled down and heat of the electronic components is diffused to the plate or the like to suppress excessive increase in temperature of the electronic components. Moreover, the heat diffused to the plate or the like is further diffused in the electronic equipment or discharged outside the electronic equipment if necessary.
When bringing the heat conductor into contact with a specific electronic component for the purpose of cooling, it is desired to increase the volume of the heat conductor to enlarge the heat capacity thereof and to increase the area of the heat conductor which is brought into contact with the electronic component to increase the speed of transferring heat from the electronic component. However, because minimization of electronic components have been advanced in recent days, the contact area of such components relative to the heat conductor is limited, and use of the cooling unit having a large volume is impossible.
A method for enhancing heat diffusion by attaching heat pipes to the heat conductor has been, therefore, proposed. Working liquid that repeatedly evaporates and condenses is sealed inside the heat pipe, and heat generated from the electronic component is transferred to an evaporation part of the heat pipe. The evaporated working liquid is then moved to a condensation part to condense in order to discharge heat. Excellent heat dissipation can be realized because the speed of the working liquid is extremely high.
FIGS. 14 through 16 show an example of a conventional cooling unit utilizing such heat pipes. FIG. 14 is a plan view showing a conventional cooling unit; FIG. 15 is a partially enlarged cross-sectional view taken along line A--A of FIG. 16; and FIG. 16 is a front view of the cooling unit. This cooling unit constitutes heat pipes 1 each of which has a flat cross section and has an outer diameter of approximately 2 mm in the vertical direction that transverses the length of the pipe and an outer diameter of approximately 4 mm in the horizontal direction that transverses the length of the pipe, a metal block 2 attached to an evaporation part 10 of each heat pipe 1, and radiation fins 3 disposed to a condensation part 11 of each heat pipe 1. As for the metal block 2, aluminum or aluminum alloy is generally used for reducing the weight and size of the cooling unit. Attachment of the heat pipe 1 to the metal block 2 in the evaporation part 10 is achieved by forming a pipe insertion hole 21 slightly larger than the flat heat pipe 1 in the metal block 2 in the direction of the thickness and inserting the flat heat pipe 1 into the pipe insertion hole 21 as shown in FIG. 15. Soldering metal 20 is subsequently poured into a gap between the surface of the heat pipe and an inner wall of the insertion hole 21 for integration.
In this-configured cooling unit comprising heat pipes, the main back surface of the metal block 2 is brought into contact with each heat generation component 5 such as an LSI on a printed board 7 through a high heat conductive rubber 6 having a good heat conductivity, and the metal block 2 is attached to the printed board 7 in this state. Heat generated in the heat generation components 5 heats the evaporation part 10 of each heat pipe 1 to evaporate the working liquid sealed inside the pipe 1. This increases the vapor pressure in the evaporation part 10 of the heat pipe 1 so that a vapor flows toward the condensation part 11 where the pressure is low. Heat from the vapor moves to the condensation part 11 and is transferred to the radiation fins 3 and diffused in the air. Accordingly, it is possible to obtain a relatively-small cooling unit having an extremely-high radiation performance.
The metal block 2 and the evaporation part 10 of each heat pipe 1 in the above-described cooling unit are provided with heat pipes that are fixed by means of the solder alloy 20 as mentioned above. However, when the material of the metal block is aluminum or aluminum alloy, an oxide film forms on the surface of the metal block preventing the soldering metal from being attached thereon and a void or bubble is generated between the surface of the heat pipe and the inner wall of the insertion hole, and the heat resistance between the heat pipe and the metal block becomes large, thereby lowering the cooling characteristic of the unit. Furthermore, the pipe insertion hole must be made large to increase the amount of the solder to be poured therein in order to suppress generation of the void and fix the heat pipe in the pipe insertion hole. In this case, because the large specific gravity of the solder metal increases the weight of the cooling unit and enlarges the insertion pipe, the thickness of the metal block must also be increased, and reduction in the thickness of the cooling unit can not be achieved.